1. Field of the Invention
The present invention relates to a fuser configured to fix an unfixed toner image on a recording material, and an image forming apparatus including the fuser. The image forming apparatus variously includes a copying machine, a printer, a facsimile machine, and a multifunction printer having integrated their functions.
2. Description of the Related Art
In recent years, an electrophotographic image forming apparatus is expected to reduce a warm-up time (a time required for copying or printing from turning on power) or reduce power consumption, from a viewpoint of energy saving. To satisfy such a requirement, a fusing belt or fusing roller to be heated has been configured to have a small heat capacity, or to have small heat loss as much as possible.
For example, JP 2009-109673 A discloses a fuser configure to completely separate the fusing belt from the pressure roller during warming up, in order to prevent excessive heat loss of the fusing belt by a pressure roller. In the fuser of JP 2009-109673 A, a pressure roller is rotated by power from a driving source. A fusing belt-driving roller receiving transmission of a rotational force of the pressure roller is brought into pressure-contact with an outer peripheral surface of a sheet non-passage area on each longitudinal end side of the fusing belt.
during warming up where the fusing belt and the pressure roller is separated from each other, the rotational force of the pressure roller is transmitted to the fusing belt-driving roller, and the fusing belt is driven to be rotated by frictional resistance with the fusing belt-driving roller. During passage of sheet, the fusing belt and the pressure roller are brought into pressure-contact with each other to form a fusing nip, so that the fusing belt is driven to be rotated by frictional resistance between the pressure roller (including a recording material passing through the fusing nip) and the fusing belt-driving roller to the fusing belt. That is, the fusing belt is driven to be rotated, and is heated along the circumferential direction (rotational direction) from a heat source. It is noted that the fusing belt has an inner peripheral surface disposed with a pressing pad making pressure-contact with the pressure roller through the fusing belt.
A fusing belt or a fusing roller needs to be heated uniformly along the circumferential direction (rotational direction), from a viewpoint of fusibility. However, in the fuser of JP 2009-109673 A, even while the fusing belt and the pressure roller are separated from each other, the pressing pad always makes pressure-contact with the inner peripheral surface of the fusing belt in the sheet passage area, and the fusing belt-driving roller always makes pressure-contact with the outer peripheral surface of the fusing belt in the sheet non-passage area, so that heat of the fusing belt is partially lost by the pressing pad and the fusing belt-driving roller.
When the fuser is configured as described above, heat loss by the pressure roller is inhibited, but efficient temperature rise may be deteriorated as much as heat loss by the pressing pad and the fusing belt-driving roller, the warm-up time or the first copy output time (FCOT) maybe increased, and power consumption may be increased.
Further, as described above, the pressing pad always makes pressure-contact with the inner peripheral surface of the fusing belt in the sheet passage area, and the fusing belt-driving roller always makes pressure-contact with the outer peripheral surface of the fusing belt in the sheet non-passage area, thereby the fusing belt wears easily, and the fusing belt is expected to be improved also in durability.